1. Field
This disclosure relates to the field of detecting and recording low-level light signals with increased resolution and precision.
2. Description of Related Art
Time-correlated single photon counting, or TCSPC, is based on the detection of single photons of a periodic light signal, the measurement of the detection times, and the reconstruction of the waveform from the individual time measurements. TCSPC makes use of the fact that for low-level, high-repetition rate signals the light intensity is usually low enough that the probability to detect more than one photon in one signal period is negligible. For each pulse of source light, the delay of the first photon that is reflected or emitted is determined and recorded. For the periodic light signal, many photons will be detected at varying time intervals. These detections can then be constructed into a histogram representing the distribution of photon probability over time.
FIG. 1 depicts a time-correlated single photon counting system known in the art (see also The bh TCSPC Handbook, Becker & Hickl GmbH, Berlin, Germany, 2nd Edition, 2006). In FIG. 1, a detector 18, typically a photo multiplier tube (PMT), delivers pulses for individual photons of the repetitive light signal that are emitted or reflected from a sample. A Constant Fraction Discriminator (CFD) 4 is used to trigger on the pulses from the detector. The CFD 4 triggers at a constant fraction of the pulse amplitude, thus avoiding pulse-height induced timing jitter. Typically, the CFD 4 triggers at the baseline transition of a re-shaped pulse, which is equivalent to constant fraction triggering.
A second CFD 2 is used to obtain a timing reference pulse from the light source used to illuminate the sample. The reference signal is usually generated by a photodiode, or, if nanosecond flashlamps are used as the light source, by a PMT operated at medium gain. The reference pulses may have some amplitude fluctuation or amplitude drift. The use of a CFD 2 in the reference channel prevents these fluctuations from causing timing jitter or timing drift.
The output pulses of the CFDs 2, 4 are used as start and stop pulses of a time-to-amplitude converter (TAC) 6. The TAC 6 generates an output signal proportional to the time between the start and the stop pulse. Conventional TACs use a switched current source charging a capacitor. The start pulse switches the current on, the stop pulse off. If the current in the start-stop interval is constant, the final voltage at the capacitor represents the time between start and stop. Conventional TACs can provide for the resolution of time differences of up to a few picoseconds.
The output voltage from the TAC 6 is sent through a Biased Amplifier (AMP) 10. The amplifier 10 has a variable gain and a variable offset. It is used to select a smaller time window within the full-scale conversion range of the TAC 6. The amplified TAC signal is fed to an Analog-to Digital Converter (ADC) 12. The output of the ADC is the digital equivalent of the photon detection time. For optimum operation, the ADC should work with an extremely high precision. Preferably, the ADC 12 resolves the amplified TAC signal into thousands of time channels that have the same width. Any non-uniformity of the channel width results in a systematic variation of the numbers of photons in the channels, creating noise or curve distortion.
The ADC 12 output is used as an address word for a measurement data memory 14. When a photon is detected, the ADC 12 output word addresses a memory location corresponding to the time of the photon. By incrementing the data contents of the addressed location using the Adder 16, a histogram of the photon distribution over time is created.
Additional details regarding prior art time correlated single photon counting systems may be found in U.S. Pat. No. 6,342,701, “Time correlated photon counting,” to Kash, dated Jan. 29, 2002; U.S. Pat. No. 6,596,980, “Method and apparatus to measure statistical variation of electrical signal phase in integrated circuits using time-correlated photon counting,” to Rusu, et al., dated Jul. 22, 2003; and The bh TCSPC Handbook, Becker & Hickl GmbH, Berlin, Germany, 2nd Edition, 2006.
U.S. Pat. No. 7,593,098, “High Dynamic Range Photon-Counting OTDR,” to Brendel, dated Sep. 22, 2009 describes the operation of an optical time domain reflectometer in a gated mode. In Brendel, a gate width circuit is used to control the time during which a photodetector is activated. A position circuit is used to position the activation of the gate to allow a specific position in the fiber under analysis to be observed. Brendel discloses that the gate widths and positions are under the control of an operator to allow for controllable observation of specific positions of a fiber and to allow for finer observations of selected portions of the fiber. Brendel also discloses the use of an optical attenuator to avoid saturation of the photodetector. Even though saturation of the photodetector is avoided, Brendel discloses the operation of the photodetector in a manner in which multiple photons are detected to allow for diagnosis of the fiber under test.